The present invention relates generally to a shift control system and method for automatic transmissions, and more particularly, to a shift control system and method for automatic transmissions of the direct-operated valve type wherein working oil pressures for respective friction elements are controlled individually.
In automatic transmissions, a power transfer route of a gear transmission mechanism or a gear position is determined by selectively hydraulically actuating or engaging friction elements such as clutch and brake. Rotation input from an engine is converted at a ratio responsive to the selected gear position to produce an output. In short, the shift is achieved by changing one gear position to another through switching of the friction elements to be engaged.
There are automatic transmissions of the direct-operated valve type wherein working oil pressures for respective friction elements can be controlled individually by solenoid valves or the like without using any buffer such as accumulator. In the transmissions, transient control of the working oil pressures for friction elements whose state is to be switched at the shift is carried out freely without being affected by the other friction elements. Moreover, due to no use of a buffer such as accumulator, transient control of the working oil pressures for friction elements can be carried out finely in accordance with the degree of shift development without being affected by the state of the buffer. Thus, the transmissions can achieve excellent shift quality in conformity of the requirements.
It is proposed to determine the degree of shift development in accordance with an effective gear ratio expressed by the ratio of transmission input rpm to transmission output rpm (=vehicle speed). Specifically, shift control is determined in accordance with inertia-phase start determination that the effective gear ratio starts to change from a gear ratio before shift to a gear ratio after shift, or inertia-phase finish determination that the effective gear ratio reaches the gear ratio after shift, or a value of the effective gear ratio during inertia phase. It is understood that, in all cases, the degree of shift development is determined in accordance with at least a vehicle-speed detected value.
However, at the shift in the vicinity of zero vehicle speed, remarkably degraded detection accuracy of the vehicle speed, etc. cause lowering of the effective gear ratio obtained as will be described later, making inaccurate determination of the degree of shift development which is carried out in accordance with the effective gear ratio. This may result in no determination of the degree of shift development which already reaches a value where control should be switched to a next phase. Thus, the automatic transmissions of the direct-operated valve type cannot achieve excellent shift quality in conformity of the requirements, losing the biggest advantage thereof.
It is, therefore, an object of the present invention to provide a shift control system and method for automatic transmissions, which allows achievement of excellent shift quality in conformity of the requirements.
The present invention generally provides an automatic transmission, comprising:
a plurality of friction elements, the friction elements being selectively engaged at shift through respective oil pressures controlled individually to determine a selected gear position; and
a controller that controls the oil pressures for the friction elements whose state is to be switched,
wherein the controller carries out a first transient control in accordance with a degree of shift development determined based on at least a vehicle speed, and
wherein the controller carries out instead of the first transient control a second transient control in accordance with an elapsed time from when the vehicle speed is too low to provide determination of the degree of shift development.
One aspect of the present invention is to provide a method of controlling an automatic transmission with a plurality of friction elements, the friction elements being selectively engaged at shift through respective oil pressures controlled individually to determine a selected gear position, the method comprising:
controlling the oil pressures for the friction elements whose state is to be switched;
carrying out a first transient control in accordance with a degree of shift development determined based on at least a vehicle speed; and
carrying out instead of the first transient control a second transient control in accordance with an elapsed time from shift when the vehicle speed is too low to provide determination of the degree of shift development.
Another aspect of the present invention is to provide a system for controlling an automatic transmission with a plurality of friction elements, the friction elements being selectively engaged at shift through respective oil pressures controlled individually to determine a selected gear position, the system comprising:
means for controlling the oil pressures for the friction elements whose state is to be switched;
means for carrying out a first transient control in accordance with a degree of shift development determined based on at least a vehicle speed; and
means for carrying out instead of the first transient control a second transient control in accordance with an elapsed time from shift when the vehicle speed is too low to provide determination of the degree of shift development.